This invention relates to radioactive well logging and more particularly to an improved method and system for testing and calibration of a radioactive well logging tool.
Perhaps the most widely used of the radioactive logging procedures are the neutron logging techniques in which a formation under investigation is irradiated with neutrons and the resulting secondary radiation measured in order to characterize the formation. The neutron irradiation may be by means of a continuous source or a pulsed source, and the secondary radiation detected typically will take the form of thermal or epithermal neutrons or gamma-rays such as may result from inelastic scattering reactions or neutron capture. In pulsed neutron logging, the formation is bombarded with repetitive time-spaced bursts of fast neutrons, and the resulting secondary radiation is measured at selected time intervals, normally by gating the output of the detector, in order to arrive at a decay parameter.
In U.S. Pat. No. 4,180,729 to Givens, U.S. Pat. No. 4,180,730 to Givens and Mills and U.S. Pat. No. 4,209,694 to Mills, there is disclosed a borehole logging system which cyclically irradiates a formation of interest and suspected of containing uranium with bursts of fast neutrons. Both epithermal and thermal neutron fluxes resulting from the irradiation of the formation are measured. These neutron fluxes are measured during that time within each cycle of operation when prompt neutrons resulting from the thermal neutron fission of uranium 235 are expected. These measurements of epithermal and thermal neutron fluxes are compared as an indication of the uranium 235 concentration in the formation. This comparison is carried out by a determination of the ratio of the measured epithermal neutron flux to the measured thermal neutron flux. This ratio is proportional to the uranium 235 concentration, provided the epithermal and thermal neutron flux detectors have the same spacing with respect to the neutron source and further provided that the neutrons detected by these detectors are counted during the same time period. In this aspect, the epithermal and thermal neutron flux detectors are concentrically mounted with respect to each other about an axis parallel to the borehole wall so as to be equally sensitive to the epithermal and thermal neutron flux die-away measurements of the formations surrounding the borehole. The thermal neutron flux detector includes a plurality of detectors concentrically spaced about the epithermal neutron flux detector, each of such plurality of thermal nuetron flux detectors having their outputs joined together. Each of the plurality of thermal neutron flux detectors and the epithermal neutron flux detectors are of cylindrical form having axes parallel to the borehole wall.